Page 96 - August 2021

EQUINE HEALTH
HYDROPS IN PREGNANT MARES
by Heather Smith Thomas
On rare occasion, a pregnant mare develops an increased amount of fluid within her placental membrane com-
partments. Her abdomen suddenly becomes much larger than you would expect for her stage of pregnancy. Karen Wolfsdorf, DVM, DACT, Hagyard Medical, explains that there are two different scenarios or two different causes for the increased fluid build-up.
The normal volume of allantoic fluid
in the near term mare is 8-15 liters. Hydroallantois is a sudden increase in the amount of allantoic fluid in the allantoic cavity, or “water bag” within the placenta, over a period of 10 to 14 days. This is believed to be due to a placental problem. It is more common than hydroamnios, which is an excessive accumulation of amniotic fluid in the amniotic sac that surrounds
the fetus. Hydroamnios develops more gradually, building up over several weeks
to months. It is often associated with a deformed fetus, with facial, genetic or con- genital abnormalities. Both usually occur in the last trimester of gestation.
“There are some recent research studies done at the University of Kentucky looking these conditions,” says Wolfsdorf. “In hydrops allantois, the main pathological finding is increased edema of the placenta. This is believed to be due to a significant decrease in capillary (blood vessel) density. The decrease in the number of vessels is associated with an abnormal expression of angiogenic, or blood vessel forming, genes. The disrupted vessel development and hypoxia, or lack of oxygen, leads to further increase in edema and extra fluid accumulation.” The fetus will be compro- mised and may not survive.
The normal volume of amniotic fluid in mares near term is about three to seven liters and composed of secretions of the amnion and from the nasal passages of the fetus, fetal saliva, seep- age of fluid/serum from maternal blood vessels, and fetal urine. “With hydrops amnios there
is much more of this fluid than normal. It has been suggested that the fetus actively regulates the volume and composition of the amniotic fluid and the prevention or impairment of swal- lowing may lead to hydroamnios,” she says.
SIGNS OF TROUBLE
“The main sign with both conditions is that the mare’s abdomen becomes very large very quickly. The abdomen looks like an apple—very round—too large for that stage
of pregnancy, and she may not want to move,” says Wolfsdorf. These mares may be off feed, uncomfortable, and have high heart rates. They may be depressed and may have severe ventral edema (swelling along the underside of the belly), abdominal discomfort, and labored breathing caused by all the extra pressure on the diaphragm. If the mare has great difficulty walking, she may prefer to remain lying down, especially with hydrops allantois.
“If horse owners recognize these signs early, they can have their veterinarian examine the mare and do something about it, and prevent catastrophe,” she says. The veterinarian can diagnose hydrops allantois (and sometimes hydrops amnios) by palpating the mare.”
Rectal palpation reveals a huge, taut, fluid- filled uterus. The fetus cannot be felt, and the uterus is usually tightly distended. “The fetus is so deep within all that fluid—way down in
“In hydrops allantois, the main pathological finding
is increased edema of the placenta. This is believed to be due to a significant decrease in capillary density.”
94 SPEEDHORSE August 2021